1. Field of the Invention
Embodiments of the invention relate to the field of semiconductor device fabrication. More particularly, the present invention relates to mass analysis magnets for ion ribbon beams used in an ion implanter used for semiconductor and other device fabrication.
2. Discussion of Related Art
In general, a beamline ion implanter provides an ion beam for treating a workpiece to obtain desired device characteristics. In one application, the workpiece is a semiconductor wafer where the ion beam dopes the semiconductor wafer with particular desired impurities. In other applications, the ion beam may provide for precise material modification to the workpiece. In addition to semiconductor wafers, the workpiece may also include, but not be limited to, flat panels, solar panels, and polymer substrates.
An ion implanter generally includes an ion source chamber which generates a plasma from which an ion beam is extracted by an extraction electrode assembly. The ion beam may then be directed towards a mass analysis magnet configured with a particular magnetic field such that only the ions with a desired charge to mass ratio travel through the mass analyzer. The mass analyzed ion beam may then be manipulated by other beam line elements known to those in the art including a corrector magnet and acceleration and deceleration lenses to direct the beam towards a surface of the workpiece. The ion beam may be distributed across the surface of the workpiece by ion beam movement, workpiece movement, or a combination of the two. The ions lose energy when they collide with electrons and nuclei in the workpiece and come to rest at a desired depth within the workpiece based on the acceleration energy.
FIG. 12 illustrates a conventional mass analysis magnet 1 having a pair of pole pieces 2 that define a gap 3 through which an ion beam 4 passes. The two pole pieces 2 create a dipole magnetic field that provides a bending force to the ion beam 4 traveling through the gap. The amount of bend varies slightly for different ion species of the beam depending on the charge state, energy, and mass of the ions. The magnetic field created between the pole pieces 2 of the conventional magnet is parallel to the direction of image/focus formed by the magnet. The mass of an ion relative to the charge of the ion affects the degree to which the ion is accelerated transversely by the magnetic field formed between the two pole pieces. Thus, as the ion beam from the ion source travels through the conventional mass analysis magnet, different ion species travel different trajectories and the mass analysis magnet selects the ions having the trajectories associated with the desired mass to charge ratio. A mass resolving slit positioned downstream from the mass analyzer magnet, selects the desired species (e.g., B+) while undesired species are collected by a conductive plate surrounding the mass resolving slit. The ions having a slightly lower ion mass (deflected through a larger angle) or ions having a slightly higher mass (deflected through a smaller angle) are not transmitted. The majority of these magnetic analyzers also orient the long direction of the ribbon beam to be parallel to the dipole field. However, the size of the ion beam accommodated by these mass analyzer magnets is limited by the size of the pole gap.
With the continuing increases in the size of some workpieces (e.g., semiconductor wafer disk sizes may increase from 300 millimeters (mm) in diameter to 450 mm and even greater) and to permit flexibility to treat workpieces of different sizes, it is advantageous to increase the width of the ion beam to a ribbon beam. A ribbon beam is an ion beam having a ribbon shape or a shape where a first dimension of the ribbon beam along one direction is larger than a second dimension of the beam along a second direction orthogonal to the first direction. The ribbon beam may have a generally rectangular cross sectional shape where the width of the ribbon beam is at least three times greater than its height.
The conventional mass analysis magnet that bends different ions at different trajectories has a limited magnetic field strength which decreases as the magnet pole gap increases. In other words, the magnetic field is inversely proportional to the width of the beam. The uniformity of the field across the width of the beam is also degraded by increasing the pole gap. Since the amount of bend varies slightly for different ion species of the beam, the non-uniform magnetic field affects the mass analysis of the ion trajectories as they travel through the magnetic field, thereby causing undesirable mass analysis results. In order to accommodate wider ribbon beams of the same mass and energy, this conventional mass analysis magnet would require a larger bend radius and hence a longer path length. This results in a larger physical magnet requiring more power supply voltage to drive a desired current through the magnet to provide a particular magnetic field strength across the gap. However, at a certain size the magnet becomes saturated and the magnetic field remains non-uniform. Consequently, the size, cost, and power consumption of a conventional mass analysis magnet would increase. Another type of mass analysis apparatus may be found in U.S. Pat. No. 6,498,348 which employs an array of elongated magnetic poles where the magnetic poles are configured in a plane perpendicular to an ion beam moving parallel to an array elongation direction. Thus, the magnetic field is perpendicular to both the width of the ribbon beam (X axis) and the direction of the beam image (Z axis). However, this type of magnet is wound around the height (Y Axis) of the ribbon beam traveling through the magnet and requires large sideways motions of the ribbon beam in order to operate. This increases the size of the magnet and also the distance that the ion beam must travel leading to increased losses in ion beam current.
Accordingly, there is a need in the art for an improved mass analysis magnet which scales well with increasing ribbon beam widths that overcomes the above-described inadequacies and shortcomings. There is also a need in the art for a mass analyzer magnet architecture that provides a uniform magnetic field which remains invariant across the width of a ribbon beam.